In the last episodes (check here), we explored different models of quantum computation. We saw that there is not just one way to build a quantum computer. There are circuit based models, measurement based models, and adiabatic or annealing based approaches. Remarkably, all of these are equivalent in what they can compute, at least in principle.

This naturally leads to a question that almost everyone asks at some point.

If we know how to build quantum computers, why do we not already have them?

There are many reasons, far too many to list in a single article. But there is one major roadblock that dominates almost every discussion in quantum computing today. Before we get to it, we need to understand a few essential ingredients.

As we discussed in episodes #2 and #3, classical computers (the computers we use today, such as smartphones and laptops, are called classical computers) work by encoding information as binary numbers, zeros and ones. Any complicated task you perform on your laptop, whether it is watching a video or running a simulation, is ultimately broken down into simple logical operations such as AND, OR, and NOT.

Quantum computers follow the same philosophy. Instead of classical bits, we use qubits. Instead of classical logic gates, we use quantum gates (learn about quantum gates here). And just like in classical computing, any complicated computation can be decomposed into a long sequence of simple, universal building blocks.

This idea is extremely powerful. It means that we do not need a special machine for every problem. We just need a small set of basic gates.

But there is a catch.